Currently the focus of mobile communication systems is on mobile broadband to provide Internet connectivity. Optimizations in such networks are typically done to improve throughput performance. These networks, and more importantly, the applied transmission schemes, are not optimized for extreme reliability and at the same time extreme latency-critical applications. Reliability in this context refers to the probability that a message, or packet, sent by a transmitter node reaches the receiver within some time interval in such a way that the receiver can decode the message.
Examples of applications requiring high-reliability and low-latency are various machine type communications, MTC, traffic safety related applications, certain smart grid applications such as tele-protection, and process control in industries.
LTE, High Speed Packet Access, HSPA, and other modern wireless communication systems apply re-transmission schemes to improve the reliability of the system.
In a simple Automatic Repeat Request, ARQ, system a transmitter transmits a message and re-transmits the message if no acknowledgement, ACK, is received within a specific time. Classical ARQ schemes do not apply Forward Error Correction, FEC, but only Error Detection to enable the receiver to determine correctness of the message. An illustration of ARQ is shown in FIG. 3, where a first (1st TX) and a second (2nd TX) transmission of a message are not successfully received by a receiver, which sends a negative acknowledgement NACK. The third transmission (3rd TX) is successfully received and acknowledged by the receiver with an ACK.
Hybrid ARQ, HARQ, is a combination of ARQ and FEC. Due to the FEC the receiver has the possibility to recover a message even if it is corrupted as long as the message is not too disturbed. If the receiver fails to recover the message it will send a negative acknowledgement, NACK, to the receiver to request a re-transmission, otherwise an ACK. Depending on the applied HARQ scheme, re-transmissions of the message are either identical copies of the original transmission or different subsets of coded bits of the original transmission. The latter is called Hybrid ARQ with incremental redundancy, and is applied in LTE and HSPA.
Hybrid ARQ can optionally use soft combining, where the receiver combines the previously received data bits and the current ones to decode the message. Soft combining is used in, for example, high speed downlink packet access, HSDPA, networks and LTE.
The minimum time between two transmissions/retransmissions is given by the time needed by the receiver to receive a transmission, decode it, generate and send the acknowledgement, either ACK or NACK; and the time needed by the transmitter to receive and decode the acknowledgement and prepare a re-transmission. This time is referred to as the Re-transmission Round Trip Time, RTT.
In LTE, the Transmission Time Interval, TTI, i.e. the minimal time unit seen by higher layers, is 1 ms. The Hybrid ARQ roundtrip time in LTE UL is fixed to 8 TTI, i.e. 8 ms.
For low-latency applications, there is a limited time within which a packet or message must be delivered, i.e. only a certain delay D is allowed for each package. Therefore, in low-latency applications, a successful reception of a packet or message must take place within this limited time or delay; otherwise the message is outdated and lost. It is a problem to achieve reliable delivery, i.e. reception, of low-latency related messages within the limited time or delay D when using regular re-transmission schemes.